For marine seismic surveying, a streamer of hydrophone receivers is towed through the area of survey, and a series of seismic shots are set off to produce downgoing source seismic wavefields. These source seismic signals propagate through the water into the sub-bottom earth and reflect from rock strata, producing an upgoing acoustic pressure wavefield in the water that in turn reflects from the water surface to produce a downgoing acoustic ghost wavefield.
For each seismic shot, the hydrophones thus receive both (a) a useful upgoing acoustic pressure wavefield caused by reflections of the downgoing source seismic signals from rock strata, and (b) the extraneous downgoing surface reflection ghosts of the upgoing wavefield. The upgoing wavefield constitutes useful seismic (acoustic pressure) data that is recorded and processed to provide a seismic survey, while the downgoing ghost wavefield constitutes noise that degrades the signal-to-noise ratio.
This signal-to-noise problem caused by ghosting cannot be ameliorated merely by adjusting the depth of the hydrophone streamer. While the magnitude of the downgoing ghost wavefield can be reduced by reducing streamer depth, that approach is disadvantageous because it increases sea state noise.
The marine seismic survey geometry using underwater hydrophones has a land environment counterpart in a technique for land seismic surveying using buried geophone receivers, which is the subject of the referenced related application titled Multicomponent Seismology Using Buried Geophone Receivers. That related application describes seismic surveying using buried receivers to improve the signal-to-noise ratio and bandwidth for shear wave seismic data by decreasing the effect of extraneous surface waves that propagate in the near surface (low velocity) region. A disadvantage of that seismic survey technique is that, in addition to receiving the upgoing reflected seismic (elastic P and S) wavefields, the buried geophones also receive extraneous downgoing surface reflection ghosts. Again, this signal-to-noise problem caused by ghosting is not ameliorated merely by reducing buried receiver depth, since the receivers are normally desired to be buried to a depth below a substantial portion of the near surface region through which the surface waves propagate.
For both marine and buried-receiver land seismic surveying, the extraneous downgoing ghost wavefields decrease the geologic resolution available from processing the seismic data to create an accurate seismic survey. Since these downgoing ghosts cannot be easily avoided, the signal processing problem is to separate the up and downgoing seismic wavefields so that the downgoing ghost wavefield can be filtered or otherwise suppressed, thereby improving signal-to-noise ratio.
The ghosting problem is particularly significant for marine seismology, which is based on using sub-surface hydrophones. However, this problem also reduces the utility of buried-receiver land seismology and its attendant advantages in improving signal-to-noise ratio and bandwidth for the elastic shear wave component of the seismic data.
Accordingly, for both marine and buried-receiver land seismology, a need exists for a seismic signal processing technique for separating useful upgoing reflection wavefields from extraneous downgoing ghost wavefields, thereby improving the signal-to-noise ratio for the seismic data and improving geologic resolution for the seismic survey.
Various seismic signal processing techniques have been used to separate upgoing and downgoing seismic wavefields, both for marine and land-based buried receiver seismology. For example, for marine seismology, one prior technique involves towing two vertically displaced streamers to provide a vertical receiver array, and uses wavefield extrapolators for both the upgoing and downgoing waves to separate the downgoing wavefield. See "Marine Seismic Exploration Using Vertical Receiver Arrays: A Means for Reduction of Weather Downtime" by M. Brink, Abstracts of the 49th Meeting of the European Association of Exploration Geophysicists, June, 1987. See, also, "2D-Deghosting Using Vertical Receiver Arrays" by L. Sonneland, et al., Abstracts from the 48th Meeting of the European Association of Exploration Geophysicists, June, 1986; and "A New Method for Separating Wave Fields into Up- and Downgoing Components" by L. Sonneland, Abstracts from the 47th Meeting of the European Association for Exploration Geophysicists, June, 1985.
For buried-receiver land seismology, the requirement for separating up and downgoing wavefields is present in vertical seismic profiling (VSP) where a vertical array of geophones detects both up and downgoing wavefields which are separated by observing, on the basis of moveout, the negative slope of the downgoing wavefield and the positive slope of the upgoing wavefield.
These previously developed signal processing techniques for separating up and downgoing seismic wavefields all require a knowledge of the measured seismic signal (acoustic pressure or elastic displacement), and the vertical gradient of the seismic signal at the seismic receiver depth. For the marine environment, the acoustic pressure gradient in the vicinity of a hydrophone is the partial derivative of pressure with respect to distance, while for the land environment, the elastic displacement gradient is the partial derivative of displacement with respect to distance. These gradients can be determined using seismic signal measurements from receivers at different depths.
The practical disadvantage of these seismic signal processing techniques for separating the up and downgoing seismic wavefields is that, in normal marine and land seismic exploration, pressure is measured by sub-surface hydrophones and displacement is measured by buried geophones, but the corresponding gradients are not available. Thus, these techniques are limited to acquisition geometries in which gradient information can be developed--for instance, marine seismology using a two streamer vertical array, or land seismology using multiple downhole receivers to provide a vertical array. Unfortunately, acquisition geometries requiring vertically displaced hydrophones/geophones often increase the expense and level of effort required for seismic surveying beyond that which is practical.